The present invention relates in general to vehicle transmissions and, more particularly, to an on vehicle apparatus to determine transmission gears engageable on a vehicle to achieve a predetermined amount of engine braking effect as such vehicle traverses a downhill grade of a roadway.
During travel in hilly areas of a country, it often happens that the driver of a commercial or utility road vehicle does not have sufficient orientation concerning what sort of uphill or downhill slope he is travelling on, because there are normally no indicators on the vehicle to provide him with this information. This can have negative consequences, especially if he has underestimated a particular downhill slope, and therefore travels thereover at an excessive speed. On vehicles with continuously variable transmissions, the generally-known rule applies, that the vehicle should travel down the hill in the gear which would be used to travel up the hill. This can also be logically applied to the speed of travel of such vehicle.
Even on commercial vehicles which are equipped with automatic transmissions, automatic assists, or shift indicators, this problem can and often does occur. The automatic mechanism, when the vehicle is travelling downhill, generally selects a gear which is too high, and then the braking action of the motor is not used to sufficient advantage.
Another problem is that the transmission ratio or the speed of travel, which is optimal when travelling downhill, is a function of both the slope and the weight of the vehicle. When the vehicle is travelling with a heavy load, a lower speed is more appropriate, so that the stopping distance in an emergency will not be too long.
A control for an automatic transmission in which the transmission ratio of the transmission is increased when the brakes are applied is described in German application No. DE-OS 31 39 985, the teaching of which is incorporated herein by reference. In this control scheme, the braking action of the motor during braking is taken advantage of. But a disadvantage of this arrangement is that no distinction is made between a level road and a downhill slope, nor are the steepness of the slope or the weight of the vehicle taken into consideration. In addition, a constant shifting of gears while braking is inconvenient for the driver of the vehicle.